The use of protective coatings on metal substrates for improved corrosion resistance and paint adhesion is common. Conventional techniques for coating such substrates include techniques that involve pretreating the metal substrate with chromium-containing compositions. The use of such chromate-containing compositions, however, imparts environmental and health concerns.
As a result, chromate-free pretreatment compositions have been developed. Such compositions are generally based on chemical mixtures that react with the substrate surface and bind to it to form a protective layer. For example, pretreatment compositions based on a Group IIIB or IVB metal compound have become more prevalent. Such compositions often contain a source of free fluoride, i.e., fluoride available as isolated ions in the pretreatment composition as opposed to fluoride that is bound to another element, such as the Group IIIB or IVB metal. Free fluoride can etch the surface of the metal substrate, thereby promoting deposition of a Group IIIB or IVB metal coating. Nevertheless, the corrosion resistance capability of these pretreatment compositions has generally been significantly inferior to conventional chromium-containing pretreatments.
It would be desirable to provide methods for treating a metal substrate that overcome at least some of the previously described drawbacks of the prior art, including the environmental drawbacks associated with the use of chromates. It also would be desirable to provide methods for treating metal substrate that impart corrosion resistance properties that are equivalent to, or even superior to, the corrosion resistance properties imparted through the use of phosphate- or chromium-containing conversion coatings. It would also be desirable to provide related coated metal substrates.